This invention relates to a combustor with a premixer, and more particularly to a combustor suited for use in a gas turbine, and also relates to a method of operating such a combustor.
In a combustor, for example, for a gas turbine, in order to reduce nitrogen oxides (hereinafter referred to as "NOx"), a premix combustion method is used in which fuel and air are premixed, and then this premixture is burned. This premix combustion method is superior to a diffusion combustion method, in which the combustion is effected by supplying fuel and air separately, in that the fuel concentration is kept low to thereby prevent localized high-temperature regions from being produced, thus reducing an amount of production of NOx in exhaust gas.
However, in the premix combustion method in which any localized high-temperature region is prevented from being produced, a flame can become unstable because of the absence of high-temperature regions, and is liable to be blown out. As a result, a back fire is liable to occur. The fuel-air premixture is not sufficiently mixed spatially uniformly, and therefore there has been encountered a problem that the effect of reducing NOx is not satisfactory. In order to overcome this, there have heretofore been made various proposals.
For example, Japanese Patent Unexamined Publication Nos. 1-203809 and 2-275221 disclose a construction in which a premixer is formed into a conical shape, and fuel is injected axially from a nozzle mounted on an apex of the conical premixer. Air is introduced in a direction tangential to the side wall of the conical premixer to produce a rotating or swirling stream flowing circumferentially within the premixer, thereby making the fuel-air premixture uniform. Also, an axial circulating flow is formed at an outlet of the premixer, thereby stabilizing a flame. Japanese Patent Unexamined Publication No. 4-103906 discloses a construction in which a flame stabilizer is provided at an outlet of a premixer, and an axial circulating flow is produced downstream of the flame stabilizer to thereby stabilize a flame. There is also known a construction in which a whirl flow is applied to a fuel-air premixture to produce a reverse flow in the vicinity of the center of the swirl, and hot burnt matters are held by this reverse flow, and by using this as an ignition source, flame is stabilized.
A combustor is required to have a compact size, and it is necessary to maintain a flame stably over a wide range from start-up to a rated operation. In the type of gas turbine combustor in which a plurality of combustors are connected together by flame propagation pipes, it is essential that each combustor should be positively ignited at the time of start-up and that the flame should positively propagate from the start-up combustor to other combustors.
However, in the construction disclosed in the above Japanese Patent Unexamined Publication Nos. 1-203809 and 2-275221 in which the premixer is formed into a conical shape, in order to sufficiently mix the fuel-air premixture within the conical premixer and also to produce a circulating flow at the outlet of the premixer, the premixer must have a sufficiently long axial length. Therefore, the premixer cannot be of a small size. Moreover, when the velocity of flow of the fuel-air premixture decreases, the circulating flow at the outlet cannot be formed stably, so that the flame becomes unstable. Therefore, this construction cannot be applied to continuous load operation of the gas turbine. Furthermore, pressure losses at an air-inlet port and at an apex of the conical premixer are large, and therefore the efficiency of the combustor is low.
In the construction disclosed in the above Japanese Patent Unexamined Publication No. 4-103906 in which the flame stabilizer is provided at the outlet of the premixer, the flame stabilizer is exposed to hot combustion gas, and therefore it is essential to cool the flame stabilizer in order to prevent burning damage. This requires a complicated cooling structure, and the premixer cannot be of a small size.
In the type of construction in which the swirl flow is applied to the fuel-air mixture, a stagnant region where the flow velocity is zero is produced in the vicinity of the center of the reverse flow region produced by the swirling effect, and therefore the flame becomes unstable, and the blowing-out of the flame and combustion vibrations are encountered.
Furthermore, since any particular consideration is given to the mixing of air and fuel within the premixer, a satisfactory uniformity of the fuel-air premixture is not obtained, and therefore the NOx-reducing effect is low. Furthermore, in the above-mentioned conventional construction, the positive ignition of each combustor at the time of start-up of the gas turbine combustor, in which the plurality of combustors are connected together by the flame propagation pipes, as well as the positive flame propagation to other combustors at the time of increase of the output, is not taken into consideration at all.